This invention generally relates to frequency modulated (FM) communication systems. The present invention is more specifically directed to improving the audio sound quality by reducing the magnitude of audible pops and bursts of noise that occur during received signal minimums caused by Rayleigh-faded received signals. The invention is especially effective for FM systems which utilize peak deviations of less than 5.0 kilohertz (kHz).
In order to appreciate this invention, the concept of a Rayleigh-faded signal should be understood. Rayleigh fading refers to the rapid fluctuations in the magnitude and/or phase of the received signal resulting from multipath propagation. A Rayleigh-faded signal is most objectionable to a listener when the magnitude of a fade is large enough to cause a substantial momentary decrease in the received signal amplitude. This results in the listener hearing an objectionable burst of noise. A common example of Rayleigh fading occurs when a mobile radio user travels down a highway while receiving a signal having rapid and substantial magnitude fluctuations in field strength. Such fluctuations may be caused by the vehicle passing relatively nearby objects, such as telephone poles or buildings, which result in field strength variations at the vehicle's mobile antenna.
If the reduction in signal amplitude due to Rayleigh fading is sufficient to reduce the IF carrier-to-noise (C/N) ratio to less than approximately 10 dB, noise in the receiver input stages will dominate and produce a burst of noise in the audio during the signal null. On the other hand, if the mean amplitude of the signal is very large, such that reduction of the IF C/N to less than 10 dB is unlikely, a noise "pop" will be generated by the discriminator during each signal null. This noise pop is due to the signal phase modulation which occurs as the signal amplitude drops, and is sometimes referred to as "random FM".
These undesired audio responses due to Rayleigh fading become more objectionable as the peak transmitted deviation decreases and the carrier frequency increases. Deviation is a factor in determining the signal-to-noise ratio. In order to achieve a given level of audio output, more audio gain will be required in a system having smaller peak deviation. Because this greater gain also amplifies the undesired noise burst or pop, these disturbances become louder relative to the desired audio (having smaller deviation) and, hence, the disturbances are rendered more objectionable. Since wavelength decreases as frequency increases, a larger number of signal strength nulls exist at higher frequencies. Thus, these audio disturbances are more likely to be encountered by a mobile radio operating at higher frequencies.
Since the available frequency spectrum is limited, and since there continues to be an increasing demand for wireless communication channels, it is apparent that better utilization of the current available communication channels is desired. One way to increase the available channels is to divide the current channel bandwidths and provide more narrowband channels. For example, if an existing 25 kHz channel is divided in half, two 12.5 kHz channels would exist. Obviously, narrower channels require that transmitted signals occupy less bandwidth. Decreasing the peak deviation from 5 kHz to 2.5 kHz in an FM system is one way to reduce the bandwidth of the transmitted signal in order to create more communication channels. New communication channels are becoming available at higher frequencies. Thus, the likelihood of having smaller deviation systems which operate at higher frequencies makes the audio disturbances associated with Rayleigh fading a significant problem.
Various noise reduction techniques have been used to control noise bursts and reduce the overall noise level in FM radio systems. Syllabic companding, high frequency equalization, automatic gain control (AGC) circuits, noise blankers, space-diversity receivers, and various other noise reduction techniques are known. However, several unique problems arise in the implementation of techniques compatible with the audio disturbances associated with a Rayleigh-faded signal in a FM receiver--especially FM systems utilizing a peak deviation of less than 5 kHz. Moreover, there is often the requirement that any noise reduction technique used in commercial FM systems must be compatible with existing radios. For example, conventional syllabic companding, i.e., transmitter-compression and receiver-expansion, cannot be retrofit into many existing systems. More particularly, any noise reduction implementation requiring an IF envelope or received signal strength indicator (RSSI) signal cannot be added to many existing radios not providing these signals. As a further complication, FM simulcast systems present the unique requirement that the noise reduction system be transparent to both high-speed and low-speed signalling and/or coded squelch data which is sent simultaneously with the voice.
A need, therefore, exists to provide an improved audio noise reduction technique which is compatible with existing FM communication systems.